The review looks at the environmental strain of construction and possible spills, while also examining potential climate change impacts. A few weeks ago, over 40,000 people rallied in Washington, D.C. urging President Barack Obama to fight climate change by rejecting the "dirty, dangerous" pipeline.

The pipeline proposal focuses on transporting crude oil from Alberta, Canada and the Bakken Shale Formation in Montana to existing Nebraska facilities. The oil would then be transported to Cushing, Oklahoma and the Texas Gulf Coast region, although the southern portion doesn't require presidential approval because it does not cross international borders.

Based on information and analysis about the North American crude transport infrastructure (particularly the proven ability of rail to transport substantial quantities of crude oil profitably under current market conditions, and to add capacity relatively rapidly) and the global crude oil market, the draft Supplemental EIS concludes that approval or denial of the proposed Project is unlikely to have a substantial impact on the rate of development in the oil sands, or on the amount of heavy crude oil refined in the Gulf Coast area.

Oil Change International's Stephen Kretzmann argued in a press statement that "the pipeline is likely to trigger at least 450,000 barrels per day of additional tar sands production capacity. In addition, by dismissing emissions associated with petroleum coke produced by the diluted bitumen the pipeline will carry, the State Department is underestimating the climate emissions of the pipeline by at least 13 percent."

TransCanada CEO Russ Girling previously declared the Keystone XL was necessary "as North American oil production increases and having the right infrastructure in place is critical to meet the goal of reducing dependence on foreign oil." But, according to the assessment, the State Department previously projected that the midstream sector of the petroleum industry "is capable of developing alternative capacity" to move central and western North American crude even if the Keystone XL is not built. "If there were no additional pipeline projects approved," the report explains, "Rail and supporting non-pipeline modes should be capable, as was projected in 2011," of delivering Canadian crude oil to refineries.

Greenpeace's Phil Radford stated, “The State Department’s report got one thing right: we don’t need the Keystone Tar Sands Pipeline to meet America’s energy needs. And it got something very, very wrong: it is just untrue that piping oil from the Tar Sands will not have a devastating impact on our climate."

Environmentalists argue the majority of known fossil fuel reserves must remain underground to avoid raising average global temperatures above two degrees Celsius. "80 percent of those reserves," to be precise, wrote 350.org's Bill McKibben in a 2012 Rolling Stone op-ed. "We have five times as much oil and coal and gas on the books as climate scientists think is safe to burn," he argued.

As McKibben declared in a written statement, "Mother Nature filed her comments last year -- the hottest year in American history; the top climate scientists in the U.S. have already chimed in. The rest of us have 45 days to make our voices heard, and we will."